Clients follow the current key for an identity, not the historical ones. A1 was Alice’s active key and followers followed A1. When a valid lineage event shows A -> A2, the client simply updates the active pubkey for “Alice” to A2 going forward. No different from how clients already update profile metadata, relays, mutes, pins, etc. Users are following the identity, so the client treats A2 as the live key and uses it for new posts, profile info, and interactions. A1 remains only as the signer of old events. That’s the entire model.